The invention relates to a composite material part having a matrix that is mostly ceramic (a ceramic matrix composite (CMC) part), which part includes an interphase layer of aluminum-doped boron nitride interposed between the fiber reinforcement and a mostly ceramic matrix phase.
A field of application of the invention is fabricating composite materials that are useful in fabricating structural parts used in the hot portions of turbine engines, in particular aviation turbine engines, e.g. parts of turbines, of after-bodies, or of secondary nozzles.
CMC parts are known that comprise fiber reinforcement made of silicon carbide (SiC) fibers, an interphase coating of boron nitride (BN) that is present on the yarns, and a matrix that is mostly ceramic.
Fabricating such CMC material parts may include a first step during which a fiber preform of shape close to that of the part that is to be fabricated is obtained by three-dimensionally weaving SiC yarns.
During a second step, the BN interphase coating may be formed on the SiC yarns by chemical vapor infiltration (CVI). During this step, the preform is held in a desired shape by means of tooling or a shaper. The BN interphase may be formed by CVI on the SiC yarns from a reaction gas phase comprising boron trichloride BCl3, ammonia NH3, and dihydrogen H2. By way of example, the CVI process may be performed at a temperature that is relatively low, e.g. about 700° C., and at a pressure that is relatively low, e.g. about 1.3 kilopascals (kPa), in particular in order to obtain a BN interphase that provides relatively strong bonding between the yarns and the interphase. Such strong bonding makes it possible to take advantage of the capacity of SiC yarns to deform elastically in order to obtain a CMC material having a high elastic deformation limit, which is thus less likely to crack under load. The BN interphase acts as a mechanical fuse by enabling cracks that propagate towards the fibers to be deflected, thereby increasing the lifetime of the material.
In a third step, the mostly ceramic matrix is formed in the residual pores of the fiber preform comprising yarns coated by the BN interphase, so as to obtain the CMC material part.
CMC materials are inevitably subjected to cracking, which, even if it does not significantly affect the mechanical properties of the materials, can nevertheless give the surrounding atmosphere access to the core of the material. Cracks or microcracks can exist as from when the material is made, or they can appear while it is in service. Unfortunately, such materials are intended more particularly for applications at high temperature in an oxidizing atmosphere (air), in particular in the fields of aviation and space.
In order to guarantee a good lifetime, it is therefore desirable to form a barrier that prevents the surrounding atmosphere from having a corrosive effect on the fiber reinforcement or on the interphase, since otherwise the mechanical properties of the material become degraded. Boron nitride oxidizes to form a liquid oxide B2O3, which acts as a diffusion barrier against oxygen. Nevertheless, in the presence of moisture, the liquid B2O3 oxide vaporizes at high temperature in the form of volatile hydroxides HxByOz. This evaporation leads to consumption by oxidation and corrosion of the boron nitride layer. This can lead to a diminution in the mechanical properties of the material.
The invention seeks to propose a solution for improving the ability of CMC parts to withstand oxidation and corrosion at high temperature.